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Old 1st April 2009, 12:37 AM   #1
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Default B&K 500 Tube Tester Calibration

I could use some help from someone who has a copy of the calibration procedure for a B&K 500. Specifically, I need the resistor value used to set the sensitivity. The 707 uses a 6K and the 650 uses a 2K, so I don't think I can just wing it here.

I'll also need to know where (what socket and pin) to apply that resistor.

Also, I'd like to know where to apply the 2meg resistor for the "shorts pot" adjustment.


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Old 1st April 2009, 03:22 PM   #2
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Woops, I said sensitivity; I ment ZERO.

The resistor is used to set the ZERO.

Any help would be appreciated.
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Old 1st April 2009, 08:02 PM   #3
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David Campbells instructions:
Signal Voltage adjustment (Test Switch in Gm position)
Set to 1.0 volt AC at terminal 5 of the wiring block on the top section of sockets to terminal 1 (gnd), Terminal 1 is most near socket 7. The terminals are numbered 1 through 12 on the schematic. This AC voltage is adjusted by R-14 which has yellow, black and orange wires attached, and mounted left of another Potentiometer when the meter is facing you under the top section of sockets mounted to the bottom of the tester. Terminal 5 is the low bias -2.5VDC voltage terminal, i.e. at that terminal you have both a DC bias and an AC signal voltage. See the following pictures for Terminal and Pot locations.
DC BIAS Adjustment (Test Switch in Gm position)
Proper DC bias at terminals 5, 6, 7 to terminal 1 (gnd) is accomplished by adjusting the R-9 potentiometer which is positioned just right of R-14. Set a voltmeter to DC volts; connect between terminals 5, to gnd (set this voltage to -2.5VDC, terminal 6 and 7 voltage adjustment is done when terminal 5 is set.

Terminal 5 is -2.5volts, Terminal 6 is -7.5 volts, Terminal 7 is -13.0 volts
Balance Control Adjustment (Test Switch in Gm position)
Connect a 6k ohm 10 watt resistor between terminal 10 (plate) and terminal 1 (gnd) with Sensitivity Control set to 100. Adjust R-19 so that you get zero reading on the tube tester meter. R19 access hole is left of the 6AT6 tube under the top section of sockets.
Short Sensitivity (Test Switch in Short position)
I donít see any adjustment for this (No more Pots). You can check to see if itís working by inserting a 1 Meg ohm resistor between pins 2 and 4 of socket 7A and looking at the shorts lamp. This lamp must be a NE-51 for the circuit to work properly.
Gas Sensitivity (Test Switch in Gas position)
Again, I donít see any adjustment for this (No more Pots again). You can check this by inserting a 20 Meg ohm resistor between pins 1 and 2 of socket 7A and seeing the meter move to about 2000 on the black Gm scale. On my tester, the reading was about 1800.
Filament Voltage Check
I checked each of the voltages for the filaments with no tubes installed first. They were each slightly higher then the settings. I then checked the 6v setting with a 5881 tube which takes about .9 amps of heater current. I measured 6.1 volts, which indicates that the transformer is working well.

Arne K
Don't believe everything you think...
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Old 3rd June 2010, 06:18 AM   #4
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I just noticed that I never thanked you for your assistance last year. My bad; your assistance was very helpful. Thanks. I have three comments to add to the procedure above.

On my tester, there is a plastic acorn nut just above the power toggle switch. Some models have a jewel power lamp in that location instead. Removing the acorn nut reveals a pot for adjusting the gas.

If you lift the lower panel and look under the shorts lamp, you will find another pot that is the adjustment for shorts. I use a 2 meg resistor to gain a little more sensitivity to shorts.

The balance can be tested without lifting the panel by placing the 6K resistor between pins 2 and 5 of socket 7A. This technique is lifted from the 707 procedure.


Last edited by Captn Dave; 3rd June 2010 at 06:30 AM.
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Old 4th May 2011, 04:27 AM   #5
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Originally Posted by Captn Dave View Post
If you lift the lower panel and look under the shorts lamp, you will find another pot that is the adjustment for shorts. I use a 2 meg resistor to gain a little more sensitivity to shorts.
I don't see it. I see R19... for the balance adjustment. Or am I wrong? Are you seeing what I see? That pot definitely adjusts the balance on my tester. Maybe I'm wrong... Can you post a picture?
I just got a BK-500 off ebay. Not in bad shape... But I can't get anything to not test as shorted. Most of the tubes I tried to test took a few seconds or more to slowly light up the neon bulb. One of the 6L6s I tried lit up bright immediately. I think that one was bad... pretty sure the the others are good. I just think the tester is out of spec and needs to be adjusted. Has anyone found any other info on this?
If I don't get any replies soon then I will start changing caps and post the results. Could be a resistor that's drifted out of spec. No reason not to continue compiling info on these testers. Anyone who is willing to work on one of these old testers surely deserves to have it work as good a possible. As old as they are, the info should be public domain by now.

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